Conversion of oxgenates in purge from raw methanol evaporator

ABSTRACT

The invention relates to a processes comprising the steps of: in an evaporator forming a gas phase methanol rich stream from a feed stream; withdrawing a liquid purge stream from the evaporator, said liquid purge stream comprising oxygenates and water; providing the gas phase methanol rich stream to a conversion step; and adding at least part of said liquid purge stream upstream the conversion step.

The invention relates to an improved preparation process of hydrocarbonsuseful as gasoline compounds from a feed comprising methanol.

Gasoline can be produced by conversion of raw methanol, pure methanoland/or dimethyl ether. In known setups the raw methanol is evaporatedbefore being mixed with a recycle gas from the conversion process andsend to the gasoline reactor. The raw methanol contains impurities inform of water and various oxygenates such as ketones, aldehydes andhigher alcohols. It has surprisingly been shown that these oxygenatesare concentrated in the evaporator/boiler to a degree where it effectsmethanol evaporation due to an increased boiling temperature. Thislowers the vaporization effectivity in the evaporator/boiler/reboilerand thus decreases the methanol flow from theevaporator/boiler/reboiler.

Thus there is a need for a process and a plant enabling a steady gasflow from the evaporator/boiler/reboiler.

In a first aspect of the present invention is provided a process forrunning on raw methanol by avoiding building up of too highconcentrations of oxygenates with higher boiling point than methanol.

In a second aspect of the present invention is provided a process whichincreases the utilization of the oxygenates in the gasoline synthesisloop.

These and other advantages are achieved by a process comprising thesteps of:

an evaporator, boiler, reboiler or similar forming a gas phase methanolrich stream from a feed stream,

withdrawing a liquid purge from the evaporator, boiler, reboiler orsimilar said liquid purge comprising oxygenates and water,

providing the gas phase methanol rich stream to a conversion step, and

adding at least part of said liquid purge upstream the conversion step.

Thus the oxygenates and other compounds are removed from the evaporatoror similar by the purge thereby ensuring that the boiling point in theevaporator is kept within acceptable levels in order to ensure a desiredflow of the gas phase methanol rich stream. The purge is then added tothe conversion step thereby maximizing the product generation in theconversion loop as at least some of the purge oxygenates are converted.I.e. by the present process a purge is removed from theevaporator/boiler/reboiler without wasting the oxygenates in the purge,as this purge is sent to the conversion step. Moreover, this processconfiguration also has the benefit of enabling the use of raw methanolas feed, thereby avoiding costly purification.

The purge may be removed continuously or on/off for example in periodicor otherwise predetermined intervals. The amount and/or frequency of thepurge may in some embodiments be controlled based on the need in orderto maintain the flow of the gas phase methanol rich stream at a desiredlevel.

For example the conversion step can be a gasoline conversion step inwhich case the methanol rich stream is converted in presence of acatalyst into hydrocarbons stream which in several embodiments is withinthe gasoline range, such as predominantly C3-C10 hydrocarbons and water.The conversion of oxygenates in the methanol rich stream is carried outin a reactor in the presence of a catalyst being active in the reactionof oxygenates to hydrocarbons, preferably C₅₊ hydrocarbons.

A preferred catalyst for the conversion reaction may be a zeolite basedcatalyst such as ZSM-5 or similar

In various setups more than one conversion reactor is used. In thesesetups the multiple reactors are preferably arranged in parallel.

The raw product from the converter in form of a gasoline reactor maycomprise hydrocarbons in the range from C1 to C13 water and carbondioxide.

By cooling and condensation of the effluent from the converter a liquidphase of water and a liquid phase comprising a mix of gasoline and lightpetroleum gas (LPG) is obtained, referred to as raw gasoline. The rawgasoline and water may be separated from a tail gas comprising Methane,Ethane, LPG, CO₂, CO, H₂ and/or C5+, part of which is recycled to theconverter. The tail gas further may comprise inerts, light hydrocarbonssuch as methane, ethane, etc. and carbon dioxide which e.g. may be usedas fuel gas. The raw gasoline may be further processed by conventionalmeans to obtain a lower-boiling gasoline fraction and a fraction of LPG.LPG may often be regarded as mainly C3 and C4.

The recycle gas may be recycled and re-introduced into the converter.The recycle stream may be compressed and/or at one or more points duringthe flow from the separator to the converter be heated, preferably byheat exchange utilizing the heat from the effluent from the converter.

The gas phase methanol rich stream is preferably mixed into the recyclestream thereby creating a mixed stream which is introduced to theconverter.

The oxygenates in the liquid purge may comprise ketones, aldehydesand/or alcohols including higher alcohols. The liquid purge may e.g.comprise water, CO₂, Dimethyl ether (DME), Acetone, Propanol, Ethanol,Butanol, one or more higher alcohols, Formaldehyde, Acetaldehyde, methylethyl ketone and methanol.

In various embodiments the liquid purge is added to the recycle from theconversion step. As the recycle is heated the liquid purge willevaporate when e.g. sprayed into the recycle stream at points afterheating of the recycle.

The liquid purge can be added to the recycle from the conversion stepup- and/or downstream the point where the methanol rich stream is mixedwith the recycle from the conversion step. Depending on where the liquidpurge is added the heat from the recycle stream can be optimally used toensure evaporation of the liquid purge when entering the recycle streamand/or mixed stream (recycle+methanol rich stream). I.e. it may beadvantageous to add the purge to the recycle stream and/or mixed streamwhere the temperature is high, such as above 180° C. Alternatively or incombination the liquid purge can be added to the gas phase methanol richstream upstream and preferably close to the methanol mixing point inorder to utilize the heat from the hot recycle stream.

The liquid purge may be added to the recycle from the conversion step byquenching such as via a spray nozzle to evaporate the liquid in therecycle stream.

The improved process described in this invention allows to run on rawmethanol as opposed to pure (grade AA) methanol. Typically, in order toproduce pure methanol, a set of distillation steps are required afterthe methanol synthesis. This separation is highly energy intensive dueto the inherent difficulty in separating water and methanol and/or otheroxygenates like ketones, aldehydes, higher alcohols, etc. Therefore, aprocess modification which allows producing gasoline from a raw methanolfeedstock is of great advantage because it makes possible to remove thedistillation steps and thus significantly reduce the investment cost.Moreover, the energy demand is greatly reduced. By way of example, theenergy required for the methanol purification is equivalent to half theenergy demand in the gasoline synthesis loop.

It is known that in the grade AA methanol specification, there aremaximum values for acetone and ethanol. Nonetheless if no purificationstep is included, the raw methanol may also comprise aldehydes,methyl-ethyl-ketone and/or C3+ alcohols, which are not included in thespecifications.

The present process is preferably carried out in a plant comprising anevaporator, reboiler or boiler, a conversion loop, at least one methanolmixing point for adding the gas phase methanol rich stream upstream theconverter and at least one purge mixing point for adding the liquidpurge to the recycle or mixed stream of recycle/methanol rich stream.One or more of the purge mixing point may e.g. be arranged up-streamand/or downstream the methanol mixing point. The position of themethanol and purge mixing points may be chosen based on variousparameters temperature, flow and/or pressure considerations as discussedabove.

For example, the methanol rich mixing point(s) may advantageously bearranged to mix the gas phase methanol rich stream into the hot recyclestream upstream a final heating of the stream to the conversion step inorder to maintain optimal temperature control of the conversion feed.The purge mixing point(s) may preferably be arranged to ensure fullevaporation of the purge to avoid purge droplets in the system. Forexample the purge mixing point(s) is arranged where the recycle streamand/or mixed stream is hot. Alternatively one or more purge mixingpoints can be arranged to mix liquid purge into the methanol rich streama stage close to the methanol mixing point. I.e. the purge can be addedto the methanol rich stream just before the methanol rich stream isheated as it is mixed with hot recycle.

The conversion loop may comprise a conversion step, a separator andmeans for returning a recycle stream to the conversion step.

The conversion loop may further comprise one or more heaters for heatingthe recycle stream, one or more coolers and/or one or more condensersfor condensing the converter effluent.

EXAMPLE

Below are exemplary parameters for conditions and compositions in thepresent plant and process. The values are exemplary and serve toillustrate the present invention and are not to be construed as limitingto the invention.

Raw Methanol:

Temperature=140-180° C., preferably 160° C.

Pressure=18-30 barg, preferably 24.1 barg

Compound wt % Water 11.6 Carbon Dioxide 0.3 Dimethyl Ether 563 wtppmAcetone  56 wtppm Propanol 2046 wtppm  Butanol 824 wtppm Ethanol 1249wtppm  Higher alcohols 821 wtppm Methyl Ethyl Ketone  44 wtppm Methanolbalance

Evaporator:

Temperature=160-205° C., preferably 182° C.

Pressure=18-30 barg, preferably 23.8 barg

Liquid Purge:

Temperature=160-205° C., preferably 182° C.

Pressure=18-30 barg, preferably 23.8 barg

Compound wt % Water 18.7 Carbon Dioxide 6.59E−03 Dimethyl Ether  142wtppm Acetone  38 wtppm Propanol 2459 wtppm Butanol 1230 wtppm Ethanol1266 wtppm Higher alcohols 1483 wtppm Methyl Ethyl Ketone  33 wtppmMethanol balance

Methanol Rich Stream Exiting Evaporator:

Temperature=160-205° C., preferably 182° C.

Pressure=18-30 barg, preferably 23.8 barg

Compound wt % Water 11.4 Carbon Dioxide 0.3 Dimethyl Ether 576 wtppmAcetone  57 wtppm Propanol 2033 wtppm  Butanol 812 wtppm Ethanol 1249wtppm  Higher alcohols 800 wtppm Methyl Ethyl Ketone  44 wtppm Methanolbalance

Methanol Rich Stream+Recycle Before Introduction in the Converter:

Temperature=290-450° C., preferably [340-410° C.] ° C.

Pressure=18-30 barg, preferably 21.3 barg

Compound wt % Hydrogen 0.5 Water 1.7 Carbon Monoxide 9.2 Carbon Dioxide15.7 Methane 27.6 Ethane 500 wtppm LPG 24.140 Ethanol 100 wtppm Methanol10.480 Dimethyl Ether 100 wtppm Acetone  <0 wtppm Propanol 200 wtppmButanol 100 wtppm Higher alcohols 100 wtppm Methyl Ethyl Ketone  <0wtppm C5+ balance

Temperature/Pressure in the Converter:

Temperature=290-450° C., preferably 340-410° C. ° C.

Pressure=18-30 barg, preferably 21.3 barg

Stream Leaving the Converter (Converter Effluent):

Temperature=320-480° C., preferably 340-410° C. ° C.

Pressure=18-30 barg, preferably 20.0 barg

Composition

Compound wt % Hydrogen 0.5 Water 7.6 Carbon Monoxide 9.2 Carbon Dioxide15.7 Methane 27.7 Ethane 473 wtppm LPG 25.1 Ethanol 100 wtppm Methanol <0 wtppm Dimethyl Ether 100 wtppm Acetone  <0 wtppm Propanol 200 wtppmButanol 100 wtppm Higher alcohols  <0 wtppm Methyl Ethyl Ketone  <0wtppm C5+ balance

DRAWINGS

In the following the process and plant is further describe by referenceto the figures. The embodiments in the figures are exemplary and are notto be construed as limiting to the invention.

FIG. 1 shows a simplified diagram of the process and plant.

FIG. 2 shows a diagram of the process and plant indicating some optionsfor the process and plant.

FIG. 1 shows a principle diagram of the present process and plant. Thediagram shows an evaporator 1 receiving a feed 2 in form of rawmethanol. From the evaporator a gas phase methanol rich stream 3 and aliquid purge 4 are withdrawn. The methanol rich stream and the liquidpurge is mixed into a gasoline conversion loop comprising a conversionstep 5 in which at least the methanol rich stream is converted into atconverted mixture (converter effluent) comprising raw gasoline. Theconverted mixture is separated into at least a recycle stream 6 and araw gasoline stream 7. At least part of the recycle is returned to theconversion step and the raw gasoline may be send to further treatment,use and/or storage.

FIG. 2 shows options for various embodiments of the present process andplant. The base process is the same as described in FIG. 1 and for likeparts like numbers are used. The mixing point 8 where the methanol richstream is mixed with the recycle is here arranged up-steam a heater 9which helps ensure a desired temperature of the stream to the converter5. As indicated by dotted lines several converters may be arranged inparallel. The number of converters may e.g. depend on the flow in thesystem. The parallel converts may be worked one or more at a time whileone or more converters are being regenerated.

The purge mixing point 10 is here arranged downstream a heat exchanger11 heating the recycle stream and upstream the methanol mixing point 8,thus vaporizing the totality of the liquid purge. Alternative positions10 a, 10 b 10 c for the purge mixing point are indicated by dottedlines. If point 10 a is used, insufficient vaporization may underdisadvantageous parameters lead to a second phase. If point 10 b isused, a similar result to that in alternative 10 is obtained, being thedifference that a higher gas/liquid ratio goes through the nozzle. Ifpoint 10 c is used, several nozzles are required (one per converter)which may increase the operation complexity due to parallel flow but maystill be a functional and relevant alternative.

Processes and plants comprising more than one methanol mixing pointand/or more than purge mixing point are also possible setups where e.g.temperature or flow conditions renders it advantageous.

In FIG. 2 is also indicated how the effluent 12 from the converter 5 ispreferably cooled by at least a cooler 13 before being separated in aseparator 14 into the recycle stream 6, the raw gasoline stream 7 andprocess water 15. A purge 16 can be taken e.g. from the recycle streamin order to reduce the amount of inerts etc. in the system.

A pump 17 for the liquid purge from the evaporator 1 and a compressor 18for the recycle stream is also indicated in the figure.

In several embodiments one or more of the heat exchangers 9 and 11utilize the heat in the converter effluent 12 whereby the (mixed) feedto the converter is heated while the effluent from the converter iscooled before condensing and separation.

1. A processes comprising the steps of: in an evaporator forming a gasphase methanol rich stream from a feed stream, withdrawing a liquidpurge stream from the evaporator, said liquid purge stream comprisingoxygenates and water, providing the gas phase methanol rich stream to aconversion step, and adding at least part of said liquid purge streamupstream the conversion step.
 2. A process according to claim 1, whereinthe conversion step is a gasoline conversion step.
 3. A processaccording to claim 1, wherein the feed stream comprises raw methanol. 4.A process according to claim 1, wherein the oxygenates comprisesketones, aldehydes and/or higher alcohols.
 5. A process according toclaim 1, wherein the liquid purge stream is added to a recycle streamfrom the conversion step.
 6. A process according to claim 1, wherein theliquid purge stream is added to the recycle stream from the conversionstep up- and/or downstream a point where the gas phase methanol richstream is added to the recycle stream from the conversion step.
 7. Aprocess according to claim 1, wherein the liquid purge stream is addedto the recycle stream from the conversion step by quenching.
 8. A plantcomprising an evaporator or boiler, a conversion loop, at least onemethanol mixing point and at least one purge mixing point.
 9. A plantaccording to claim 8 wherein the conversion loop comprises a conversionstep, a separator and means for returning a recycle stream to theconversion step.
 10. Plant according to claim 8 wherein the conversionloop further comprises one or more heaters for heating the recyclestream, one or more coolers and condensers for condensing the convertereffluent.
 11. Plant according to claim 8, wherein one or more purgemixing points are arranged up-steam and/or downstream the methanolmixing point.
 12. A plant comprising an evaporator or boiler, aconversion loop, at least one methanol mixing point and at least onepurge mixing point, arranged to carry out the process according toclaim
 1. 13. Gasoline product produced according to the process ofclaim
 1. 14. Gasoline product produced by the plant of claim 8.